Kinyume was never really interested in (see "too shy to") getting dates for the school dances she attended. Luckily, she had a self-appointed best friend who didn't care about going as dates or as friends, as long as he got to go to a party, so Kinyume has several photos like this one in her possession, buried as deep as possible in one of her drawers.
Now, the problem with the school dances, aside from the social pressures of not going alone, was that they had a dress code; Kinyume does not like dressing up, and she will kick anyone who tried to force her into a dress. So she usually went with the abolute minimum required, in this case being a simple black suit collar.
Disco, on the other hand, loves to wear clotheshe likes to look as pretty and sparkly as he's allowed to. (He once tried to wear a dress made out of glowsticks, but he got kicked out. "It blinded the other students" were an excuse, he knew the teachers were just jealous.) So his attire were usually dresses, because they're a lot easier to find in bright, sparkly variants -there exist colorful, glittery suits, but they just aren't as pretty as the dresses.
Here, he also made sure to bring a tie and rose that matched his dress for Kinyume to wear -if they were getting their picture taken, they were gonna match, wether Kinyume liked it or not. Then they passively agressively argued about it so much, they didn't even look at the camera. Woops.
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Yeah, Disco doesn't really care about social conventions, as long as he looks pretty. Though, I'd imagine pony society would be a lot more relaxed when it comes to wearing clothing not traditionally worn by your gender -when clothing in and of itself is optional, I think what specific clothing you chose to wear might matter less? Also, with the rulers of the land being millenia year old females, femininity is probably a lot more acceptable in males than it is in our culture.
Whatever, I'm just trying to excuse that I wanted to draw Disco wearing a dress, 'cause I thought he would look cute in one. Which he did